1. Field of the Invention
The present invention relates to a method and an apparatus for driving a stepping motor, and more particularly to a method and an apparatus for driving a stepping motor incorporated into, for example, a drive system in a printing mechanism of a printer, and to improvements of a microstep drive technique for driving the stepping motor.
2. Related Art
The stepping motor is widely used in various devices and apparatuses where a rotary mechanism requires a highly precise control, e.g., printer, in particular a drive system for its printing mechanism. As well known, the stepping motor is a rotary machine of the type in which a pulsative drive current is fed to the drive coil (stator) of the motor, and a rotor of the motor is rotated at a speed defined by a pulse frequency of the drive current and at an angle defined by the number of pulses of the drive current.
One of the known drive methods of this type is a "microstep drive" method, disclosed in Japanese Patent Laid-Open Publication No. Sho-62-254696. The microstep drive method is as follows. A sinusoidal waveform is sampled at a given resolution, and the sampled amplitudes are composed to form a substantially sinusoidal waveform in which the sampled amplitudes of the waveform are stepwise chained to form a contour of the sinusoidal waveform. A waveform of the drive current fed to the stepping motor is shown in FIG. 1. A set value of each step of the waveform of the drive current is set at L1. The drive current is alternately increased and attenuated in the step, with its upper peaks being limited within the set value L1. Such a drive current is formed by chopping.
In this microstep drive method, the value of the drive current actually flowing through the stepping motor, i.e., an average value L2 of the drive current, is smaller than the set value L1 of the step. Therefore, the waveform composed by the set values is somewhat lean when compared with an ideal sinusoidal waveform. Thus, in the conventional microstep drive, a changing rate of the amplitude of the waveform is smaller than that of the ideal sinusoidal waveform in the vicinity of the zero-crossing point. This appears as an irregular rotation of the stepping motor. The irregular rotation brings about a vibration of the motor. Therefore, when the stepping motor is used for driving the carriage or platen of the printer, noise is generated when the motor rotates and the print quality of the resultant print is deteriorated.
In case where the microstep drive is used for the stepping motor, two chopping methods are used. A first chopping method attenuates the drive current relatively gently or at a small attenuating rate, and a second chopping method attenuates the drive current sharply or at a large attenuating rate.
In the first chopping method, the attenuation of the drive current is gentle. Therefore, the waveform of the drive current is distorted as a whole from an ideal sinusoidal waveform when the drive current waveform is shifted from one step to the next step. The waveform distortion causes the motor to vibrate and to generate a noise. Further, it increases a motor copper loss, so that the quantity of heat generated by the drive coils and the drive apparatus is increased.
In the second chopping method, the chopping frequency is high. Therefore, the motor copper loss is large, and hence the quantity of heat generated by the drive coils and the drive apparatus is increased. Further, the ripple component is increased, so that the motor torque is reduced.
A proposal is made to solve those problems. In the proposal, as shown in FIG. 2, the current is first attenuated at a relatively small attenuation rate Is (attenuating is relatively gentle), then it is attenuated at a relatively large attenuating rate If (attenuation is relatively sharp), then it is attenuated at the attenuating rate Is, and then it is attenuated at the attenuating rate If, and so on. The proposed chopping method insufficiently removes the adverse effect of the distortion of the drive current in a region where the set value of the drive current decreases in its absolute value. Therefore, the proposed chopping method insufficiently suppresses the vibration and noise generation in this set-value decreasing region.